The present invention relates generally to optical scanning systems. More particularly, this invention relates to a scanning system containing a camera using a coplanar light source.
Various optical scanning systems have been developed for reading and decoding coded symbologies, identification of objects, comparison of objects, and measurement of objects. Each of these scanning systems utilizes either a non-coherent or coherent light source. Lighting is one of the key elements in obtaining good image quality. The intensity of light needed for scanning is directly proportional to the transport speed of the scanned object and the speed of the sensor. Generally, the faster an image is to be acquired, the more light is needed. Until now, only high intensity sodium or halogen lighting was adequate to obtain crisp images in cameras that focus over a significant depth of field at high speeds. The light source is usually located off axis from the camera and sensor detecting the light reflected from the object being scanned.
In applications using sodium lamps as a light source, the lamps are used to provide the illumination required by the camera detection means. These lamps provide an abundance of optical power because they are very bright and have a wide spectral range. There are, however, several disadvantages to sodium lamp light sources. First, due to their extreme brightness, sodium lamps can create an annoyance and possible hazard to workers working in the vicinity of the scanning systems. Second, sodium lights require a large amount of AC power, thus increasing production costs. Third, these light sources create a large amount of heat. Additionally, radio frequency interference can be created which can present operational problems to equipment in the vicinity of the scanning system.
The use of light sources such as LEDs presents several advantages over sodium and halogen lighting. LED illumination is a more cost effective and ergonomic method of illumination. The problem presented by LED illumination is how to get enough light to the object that is being imaged when focusing over a large depth of field. By eliminating the mounting angle between the light source and the line of sight of the camera lens, the reflected light is managed and a lower intensity light source may be used. Because LEDs can be energized almost instantaneously, they can be de-energized when objects are not being transported within the field of view. This extends the life of the LEDs and also conserves power. Additionally, the power input to individual LEDs may be modulated and pinpointed to a desired area, such that different LEDs within an LED array may be energized at different levels according to the desired application.
The use of a coherent or non-coherent light source which will provide sufficient optical illumination to an object to be scanned, which uses less energy while alleviating potential problems of radio frequency interference or heat emission is needed.
Briefly stated, the present invention provides an optical scanning system which uses a light source to provide an illumination stripe that is coplanar to a camera lens and light sensor for barcode reading applications. The light source may be coplanar to the lens axis and light sensor, and preferably is formed from LEDs or other low power consumption illumination sources. The coplanar design provides adequate illumination for a large depth of field at low speeds.
In another aspect, the invention provides a scanning system in which the light source is shifted relative to the line of sight of the camera such that the illumination stripe remains coplanar with the camera line of sight at the required depth of field. The light stripe profile coming from the array can therefore be narrow. The intensity of light required to illuminate an object over the depth of field is significantly reduced, thus allowing for the use of an LED array or other low power light source.
In another aspect, the invention provides a plurality of off-axis light sources to provide an illumination stripe on the object generally coplanar with camera line of sight at the required depth of field. Different arrays of lights sources are energized according to the depth of field of the target object, allowing adequate lighting over a range of distances.